Process for integration of a chosen gene on the chromosome of a bacterium using Mu transposons

ABSTRACT

The present invention relates to a process for integration of a chosen gene or of a specific DNA molecule into the chromosome or episome of a bacterium by cloning the gene or DNA molecule into a defective transposon which is then integrated into the chromosomal or episomal DNA of the bacteria. The defective transposon is incapable of transposition autonomously but can be induced to transpose when properly complemented. The complementation to induce transposition can be limited so that the defective transposon produces a specific number of copies and thereafter is stable. The number of copies of the defective transposon produced during the period of transposition can be estimated by the level of expression of a marker gene contained within the defective transposon.

This application is a continuation of U.S. patent application Ser. No. 07/859,610, filed Mar. 23, 1992 (now abandoned) which is a continuation of U.S. patent application Ser. No. 07/313,625, filed Feb. 21, 1989 (now abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to a new method for the amplification and stabilization of the copy number of chosen genes in a microorganism.

The invention is illustrated by means of examples given hereinafter in which the sequence coding for an enzyme, beta-galactosidase, and the genes coding for three enzymes involved in the biosynthesis of L-threonine are transferred inside the chromosome of a microorganism.

These genes have been shown to be stably fixed in the chromosome. The resulting strains are therefore better producers of, in the first instance, the corresponding enzymes or, secondly, of L-threonine amino acids, than are the parental strains.

The bacteria are capable of synthesizing useful active principles such as enzymes and amino acids. The production by fermentation involves complex biochemical procedures controlled by multiple genes and gene products. The products of the fermentation are obtained in relatively small quantities. In order to improve this production by fermentation, it would be useful to increase the number of copies of the genes involved, in a manner such that the number of these copies could be controlled and stabilized in the chromosome or the episomes of the microorganism.

Several methods have been described in the scientific literature in which the copy number of a gene can be amplified in a microorganism, in particular by means of circular plasmid vectors. However, the number of copies obtained by these methods is extremely variable and depends on the particular vector, the length of the chosen gene and the particular physiological conditions of the microorganism. None of these methods are concerned with the amplification of the gene to a chosen level, avoiding an unnecessary excess of copies, and this with a stable fixation of these copies in the chromosome.

Various documents suggest the use of transposons as vectors in bacteria or other organisms, in particular:

U.S. Pat. No. 4,670,388,

EP-A-0,200,708

EP-A-0,091,723

GENE, vol. 42 (1986), pages 185-192

THE EMBO JOURNAL, vol 4, No. 4, 1985, pages 891-898, and

BIOTECHNOLOGY, vol. 4, No. 5, May 1986, pages 446-449, New York, U.S.

However, none of these documents describes a process by which it is possible to obtain a stable integration of a specific DNA sequence with a defined copy number.

SUMMARY OF THE INVENTION

One aim of the invention is to describe a method for amplifying and stably fixing the copies of chosen genes in a microorganism with more flexibility and more reliability than was hitherto possible. This aim as well as other objects of the invention, such as are envisaged hereafter, have been accomplished, for example, by providing the Escherichia coli bacterium with a transposable element having a wide host range and derived from the Mu phage, and in which the genes of the transposase cannot be expressed or are deleted. Such a transposon is a defective transposon. Under particular reversible genetic conditions which are easy to reproduce for organisms of the same genus or of a different genus, this transposon and the genes which can be introduced into it by genetic engineering technologycan be made to multiply inside the chromosome of the microorganism. When the particular genetic conditions are removed, the various copies of the transposon remain fixed in a stable manner inside the chromosome.

The present invention relates to a process for integration of a chosen gene or of a specific DNA sequence (inside a bacterium) in a DNA sequence such as a chromosome or an episome, wherein:

a) the said chosen gene or the chosen DNA sequence is cloned inside a defective transposon outside the essential parts of the transposon,

b) the said transposon is integrated in the DNA sequence inside the bacterium.

More particularly, the process according to the invention is used to amplify the chosen copy number of genes in order to obtain a specific number of copies. In this case the process according to the invention is a process for integration of a specific number of copies of a chosen gene or of a specific DNA sequence in a DNA sequence such as the chromosome or episome of a bacterium, wherein:

a) the said chosen gene or the chosen DNA sequence is cloned inside a transposon outside the essential parts of the transposon, the said transposon being defective,

b) the said transposon is integrated in the DNA sequence such as the chromosome or the episome of the said bacterium,

c) the said defective transposon is complemented so as to be able to transpose several times and the complementation is then stopped after a specific number of transpositions.

According to the invention any transposon can be used which transposes inside any site in the bacterial DNA.

In the present text the term "transposons" is intended to mean both "true transposons" and also phages having the capacity to transpose.

Preferred transposons are Tn3, Tn5, Tn7, Tn10, Tn903, TnHoHo, IS 1, IS 10, IS 50 and the MudI and MudII phages. The MudI and MudII phages are particularly preferred.

The use of defective transposons, which are transposons incapable of transposing by themselves, makes it possible to obtain a stable integration. For such a use the transposons used have no genes for the transposase or these genes are not active in the said bacterium. A transposon is very particularly preferred which lacks genes coding for the transposase, but it is also possible to use a transposon in which the trans-acting transposases are repressed or inactive under normal conditions.

According to the invention it is possible to use any bacterium in which the transposons described hereinabove can transpose.

The preferred bacteria are members of the Enterobacteriaceae family, for example Escherichia coli, Citrobacter freundii, Erwinia herbicola or E. chrysanthemi, Salmonella thyphimurium, Shigella soneii, Enterobacter cloacae, Serratia marcescens or members of the Rhizobiaceae family, such as Agrobacterium tumefaciens or members of the Pseudomonas family, such as Pseudomonas putida for example.

By the term chosen gene there is understood a gene which is not normally present in the said transposon or which is not present in the host bacterium.

As chosen gene there may be used a cloned gene or a hybrid gene, a gene segment or a synthesized gene. The gene can be expressed in the host bacterium and may be of animal, vegetable or microbial origin. Several genes can be inserted in the transposon.

The chosen gene can be under the control of a promoter which is itself inside the transposon and which ensures the expression of the said gene in the said bacterium.

The cloned genes are inserted by known genetic engineering techniques or according to the process described hereinbelow.

For convenience sake, it is simpler to start from a plasmid containing the defective transposon. The transposon is made defective by deletion or mutation.

The plasmid containing the transposon is isolated from the bacterium and cleaved with appropriate restriction enzymes. The chosen genes are also extracted from their host and then purified after cleavage by means of restriction enzymes. The cloning of a marker gene such as an antibiotic-resistant gene (npt for example) or another "reporter" gene (lacZ for example) at the same time as the inserted genes is preferable for the selection of the transposition events. The number of transpositions is evaluated by determination of the expression of the marker gene. The said expression of the marker gene can be determined by the resistance to an antibiotic when the marker gene is an antibiotic-resistant gene; the expression of the marker gene can also be determined by the coloration of a substituent when the marker gene codes for an enzymatic activity which can be evaluated by a colorimetric assay. The various fragments are recombined by ligation in vitro and transformed in the appropriate recipient bacterium. The transformants are selected and then analyzed in order to establish that the structure is suitable. The resulting composite defective transposon can be placed in other strains by transformation or transduction. Conventional genetic engineering methods are described in "Molecular cloning, a laboratory manual" (Maniatis et al., Cold Spring Harbor Laboratory, N.Y., 1986).

When the process of the invention is also used to amplify the number of copies of the chosen gene, the integration of a specific number of copies of the said chosen gene, during the process described hereinabove using a defective transposon and after integration, the said transposon incapable of transposing is complemented in order to be able to transpose several times, and the complementation is then stopped after a specific number of transpositions.

The said complementation can be carried out in various ways. If the transposon is truly defective, that is to say if it does not possess any transposase, the latter can be introduced into the host cell by means of a plasmid or a phage coding for the said transposase.

If the expression of the transposase is repressed, it is possible to inactivate the repressor. In the case of a repressor sensitive to heat an increase in temperature will result in the expression of the transposase.

Once inserted in the recipient DNA, the transposon produced can be amplified by transposition onto the genome by means of inactivation of the repressor leading to expression of the genes of the transposase.

The transposon in which the transposase genes are lacking can be amplified by introduction of a plasmid or a phage containing trans-acting transposase genes.

When the complementation is effected by introduction of an extra-chromosomal element, such as a plasmid or phage especially, this complementation can be effected at the same time as the integration stage.

The amplified transposons are maintained in a stable manner in the bacterial DNA under the normal culture conditions. It is preferred to use a defective transposon to stabilize the chosen genes. Any DNA whatsoever in a bacterial cell can be the transposition target. DNAs are very particularly preferred which are maintained in a stable manner in the bacterial cell such as the chromosome or a stable plasmid.

According to a preferred embodiment of the invention the defective transposon used is MudII1734 (Castilho et al., J. Bact., 158, 488, 1984) which is present as a defective prophage in an E. coli strain (POII1734) already lysogenic for a Mucts mutant phage. This strain, cultured at low temperature, is briefly heated and the resulting lysate is used for the transduction with a suitable recipient strain. The bacteria which are resistant to the appropriate antibiotic and which do not produce any Mucts phage only contain the transposon in their chromosome.

The MudII1734 is inserted on a plasmid, and the inside of the Mud can then be modified (according to the above part of the description page 3, line 37 to page 4 line 16).

The plasmid containing the modified Mud is transformed in a strain lysogenic for Mucts. The phage lysate is obtained after increasing the temperature. This phage lysate is used to transduce the modified Mud on the chromosome of a specific bacterium.

In order to amplify the modified Mud, the strain which carries the defective transposon on its chromosome is transformed with a plasmid which contains the two transposase genes, namely genes A and B, or is infected by the Mu phage.

In the case of amplification by a plasmid, the plasmid containing the A and B genes is maintained in the bacterium by use of a (antibiotic-resistant) selection. The bacteria which grow in the presence of this antibiotic and at the same time synthesize in large quantity the product of a gene situated in the transposon are selected. The bacteria which express these two phenotypes can contain several additional copies of the transposon on their chromosome.

The bacteria which have lost the plasmid and which are not lysogenic for the phage can be obtained naturally.

The determination of the transposon copy number can be carried out by selection of the bacteria described hereinabove according to the level of expression of a marker gene in the transposon.

The number of copies of the transposon containing an antibiotic-resistant gene can be estimated according to the resistance of the bacterium to the antibiotic.

The number of copies of the transposon including the lac operon can be estimated according to the intensity of the coloration of bacterial colonies after degradation of a substrate such as X-gal (5-bromo-4-chloro-3-indolyl-beta-D-galactoside).

The number of copies of the transposon can be determined using Southern blotting and DNA-DNA-hybridization techniques with appropriate probes.

The chosen genes amplified by means of the transposon remain stable insofar as the transposase genes are not present or are inactive in the bacterial strain.

To this end it is also preferable to introduce into the host bacterium, after amplification, DNA polymerase I or 5'-3' exonuclease mutations or an HU deficient mutation.

An important aspect of this invention is that the strains obtained are very stable. The defective transposon can neither transpose further nor be deleted by any efficient mechanism. After a culture for many generations without selective pressure, the defective transposons still occupy the same position in the chromosome.

Moreover, no transfer of the defective transposon is detectable in cocultivation experiments.

The present invention also relates to the said bacterium which can be obtained by the process according to the invention and likewise relates to a process for the preparation of the product of the heterologous gene comprising the culture of the bacterium according to the invention in a suitable culture medium and the recovery of the said product.

Although the chosen gene can code for a peptide or a protein having a therapeutic or industrial use, one of the implementations of the process concerns the preparation of metabolites by using the bacterium obtained according to the process described hereinabove using a defective transposon containing one, several or all of the genes for the biosynthesis of the metabolites.

Among the genes of industrial interest, there should be mentioned genes coding for the biosynthesis of amino acids, in particular threonine or for the biosynthesis of vitamins, such as biotin. However, the amplification of the expression of certain enzymes may also permit an overall super-expression of certain metabolites, the examples showing the super-expression of the asd gene and the aspA gene.

The process according to the invention is very versatile and can be used to amplify any type of DNA sequence.

The bacterium thus obtained, having the capacity to produce one or several enzymes, or one or several amino acids or desired products can be cultured in much the same way as usual. Ordinary culture media are used containing carbon sources, nitrogen sources, inorganic ions and, if required, organic micronutrients such as amino acids and vitamins. Suitable carbon sources include glucose, sucrose, lactose, starch hydrolysate containing these sugars, whey and molasses. Preferred sources of nitrogen include ammonia gas, ammonia water and ammonium salts. Culturing is carried out under aerobic conditions at a controlled pH and temperature until the cells in practise cease to produce the desired product.

For a production such as that of an amino acid or a protein, the strain which already produces the desired product is preferable as a host strain for the envisaged production.

The following examples and the figures will illustrate other characteristics and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: diagrammatically represents the restriction map of the MudII1734 transposon with a wide host range.

FIG. 2: diagrammatically represents the construction and the restriction map of the pBR322 (MudII-1681) and pPR30 plasmids.

FIG. 3: shows the results of electrophoresis followed by the blot hybridization of:

DNA from a strain in which the copy number of the transposon has been amplified (lane f),

DNA from 5 strains isolated after 170 generations (lanes a to e), (the probe is pCE1134; the DNA was digested by EcoRI).

FIG. 4: diagrammatically represents the construction and the restriction map of the pCE1134 plasmid.

FIG. 5: diagrammatically represents the construction and the restriction map of the pAM9 plasmid.

FIG. 6: shows the results of electrophoresis followed by blot hybridization of the DNA of strains AJ11332, EL1001, EL1002, and EL1003 respectively for lanes A to D; (the probe is a small EcoRI fragment of pAM9; the DNA of the strains was digested by EcoRI).

FIG. 7: diagrammatically represents the construction of pEV11, pMC1 and pMC23.

FIG. 8: represents the analyses of the transpositions of MudAM9 by the Southern DNA-DNA hybridization technique in strains AJl1332, EL1008 and EL1009.

FIG. 9: shows the results of the electrophoresis followed by blot hybridization of the DNA of strains AJl1332, EL1001, EL1004, EL1006, EL1003, EL1005 and EL1007 respectively for lanes A to G (the probe corresponds to the pRC100 plasmid digested by HindIII).

FIG. 10: shows the results of electrophoresis followed by blot hybridization of the DNAof strains E. chrysanthemi 384551, EL3003 and EL3007 for lanes A, B and C respectively (the probe used is the EcoRI-HindIII fragment of pAM9 plasmid).

FIG. 11: shows the results of electrophoresis followed by blot hybridization of the DNA of strains E. chrysanthemi 384551, EL3004 and EL3008 for lanes A to C respectively (the probe used is the Sal-BamHI fragment of the pAM9 plasmid.

FIG. 12: shows the construction of the pAMll plasmid containing the asd gene and the threonine operon.

FIGS. 13 to 16: show the results of electrophoresis followed by blot hybridization of the DNA of strains EL1016, EL1012 and EL1013 for lanes A to C respectively.

FIG. 13: In FIG. 13 the chromosomal DNAs are digested by SalI and are hybridized with the EcoRI-HindIII fragment containing the genes nptII, thrA, thrB and thrC' of pAM9 as the probe.

FIG. 14: In FIG. 14 the chromosomal DNAs are digested by EcoRI and hybridized with the EcoRI-HindIII fragment containing the genes nptII, thrA, thrB and thrC' of pAM9 as probe.

FIG. 15: In FIG. 15 the chromosomal DNAs are digested by SalI and hybridized with the BamHI fragment, containing the asd gene, of pAM11 as the probe.

FIG. 16: In FIG. 16 the chromosomal DNAs are digested by EcoRI and hybridized with the BamHI fragment, containing the asd gene, of pAM11 as the probe.

FIG. 17: represents the cloning of the aspA gene in a defective phage.

FIG. 18: shows the results of electrophoresis followed by blot hybridization of the DNA of strains MC4100, EL1010 and EL1011 for lanes A to C respectively.

The probe is a plasmid containing the aspA gene (the DNA was digested by SalI).

FIG. 19: shows the results of electrophoresis followed by blot hybridization of the DNA of strains EL1016, EL1014, EL1015 for lanes A to C respectively.

The probe is the SalI-BamHI fragment containing the threonine operon of the pAM9 plasmid, the DNA is digested by the HindIII enzyme.

FIG. 20: shows the results of electrophoresis followed by blot hybridization of the DNA of strains EL1016, EL1017, EL1017-a, EL1017-b, EL1017-c and EL1017-d for lanes A to F respectively (the probe is the EcoRI-HindIII fragment containing the genes nptII, thrA, thrB and thrC' of the pAM9 plasmid.

DETAILED DESCRIPTION

EXAMPLE 1: PRODUCTION OF AN ENZYME HAVING A BETA-GALACTOSIDASE ACTIVITY BY AMPLIFICATION OF THE CODING REGION RESPONSIBLE FOR THE PRODUCTION BY FERMENTATION OF BETA-GALACTOSIDASE

Preparation of a strain containing in its chomosome one additional copy of the beta-galactosidase gene cloned inside a defective transposon

The E. coli strain POII1734 : F⁻, araD139, ara :: (Mucts 3, Δ(lac)X74), galU, galK, rpsL (Sm^(R)), (MudII1734) containing MudII1734 (FIG. 1) (Castilho et al., J. Bacteriol., 158, 488, 1984) was cultured in 2 ml of LB medium for 2 hours at 30° C., was subsequently subjected to thermal shock at 45° C. for 15 minutes, and was then cultured at 37° C. for 2 hours, this leading to a phage lysate. In order to obtain the phage solution, this lysate was mixed with several drops of chloroform and subsequently centrifuged. The supernatant was used as phage solution to infect the bacterial strain.

The E. coli strain MC4100 : F-araD139, Δ(argF-lac), U169, psL150, RelA1, flb5301, ptsF25, deoCl (Casabadan, 1975) such as described by Castilho et al. (1984) was cultured overnight in LB medium. CaCl₂ and MgSO₄ were subsequently added, giving a final concentration of 1 mM and 2.5 mM, respectively. 200 μl of this culture and of the phage solution described hereinabove were mixed and left at room temperature for 15 minutes. After addition of 2 ml of LB medium and incubation for 2 hours at 30° C., this cell suspension was spread on a dish of LB medium containing 20 μg/ml of kanamycin (Km) and 10 mM of X-gal and subsequently incubated for 24 hours at 30° C. in order to select the transductants.

The colonies not producing Mucts after checking by spreading the replicas on a layer of E. coli sensitive to Mu were identified and one strain MC4100 (MudII1734) was retained. This strain does not produce beta-galactosidase, as shown by the white color of the colonies on the dishes of LB medium supplemented with X-gal, a substance which changes from colorless to blue when it is hydrolyzed to beta-galactosidase.

The DNA was extracted, cut with the EcoRI restriction enzyme and the fragments obtained were separated on a gel and blotted on nitrocellulose. After hybridization with the DNA of the labeled transposon, two bands were visible on the resulting autoradiogram indicating the presence of a copy of the transposon in the strain.

Cloning of the A and B transposase genes of the Mu phage on a multicopy plasmid

The DNA extracted from the plasmid pAB8155 containing the defective phage MudII1681 was cut with the EcoRI and EcoRV restriction enzymes. The 8.3 kb fragment which contained the A and B transposase genes was purified by gel electrophoresis and cloned in the circular vector pUC19 linearized with SmaI and EcoRI. The reconstituted plasmid was transformed in a suitable recipient bacterium. The colonies resistant to antibiotics were selected and were shown to contain the pPR30 plasmid (FIG. 2).

MC4100 (MudII1734) was transformed with pPR30 after treating the cells with calcium chloride (Mandel and Higa, J. Mol. Biol., 53, 159, 1970). After 2 hours at 30° C. in LB medium the cells were spread on dishes containing 20 μg/ml of Km, 25 μg/ml of ampicillin (Ap) and 10 μg/ml of X-gal. Incubation was carried out for 48 hours at 30° C. and colonies colored a blue of variable intensity appeared. These colonies were purified on the same medium and selected.

The total DNA was extracted from independent colonies, digested with EcoRI and then subjected to electrophoresis and transferred to a nylon membrane. After hybridization with the DNA of the radiolabeled transposon, several bands were detected on the autoradiogram indicating the amplification of the transposon. The pPR30 plasmid was eliminated from these strains by several successive transfers on LB medium without Ap.

The AM1 strain was studied in particular: after the DNA had been isolated, digested by EcoRI and subjected to electrophoresis, the DNA fragments were transferred to a nylon membrane. 4 hybridization bands can be seen in FIG. 3 (lane f) when the probe used is a radiolabeled pCE1134. These 4 hybridization bands correspond to two insertions of MudII1734.

Stability of the copy n-mher of MudII1734

The strain AM1: MC4100 (MudII1734) which has two copies of MudII1734 on its chromosome was cultured for 170 generations in LB liquid medium without Km. The copy number of MudII1734 on the chromosome was then measured by DNA-DNA hybridization (FIG. 3, lanes a to e). The copy number in this strain remained stable and no change in the restriction pattern could be detected, this indicating a high stability of the insertions.

Production of beta-galactosidase

The strain thus obtained is able to produce beta-galactosidase. The strains MC4100 (MudII1734) and AM1 produce, respectively, 7.10⁻¹ and 10 nMole/mn/mg of protein having a beta-galactosidase activity.

EXAMPLE 2: PRODUCTION OF L-THREONINE BY AMPLIFICATION OF THE GENES RESPONSIBLE FOR THE PRODUCTION OF L-THREONINE BY FERMENTATION

Construction of pCE1134

The strain POII1734 (Castilho et al., J. Bacteriol., 158, 488, 1984) was transformed with pCE100 (2.1 kb, Cm^(R), FIG. 4) after treating the cells with calcium chloride (Mandel A. and Higa M., J. Mol. Biol., 53, (1970).

The transformants were selected on dishes of LB media containing 25 μg/ml of chlorophenicol (Cm) after 24 hours of incubation at 30° C. The transformant POII1734/pCM100 was cultured for 2 hours at 30° C. in 3 ml of LB media, subsequently subjected to the shock for 15 minutes at 45° C. and then cultured at 37° C. for 2 hours, leading to a phage lysate. In order to obtain the phage solution, this lysate was mixed with several drops of chloroform and subsequently centrifuged. The supernatant was designated phage solution I.

E. coli M8820 (Mu) (Castilho etal., J. Bacteriol., 158, 488, 1984 ) was cultured overnight in an LB media to which calcium chloride and magesium sulfate were added to give a final concentration of 1 mM and 2.5 mM respectively. 200 μl of this culture and of phage solution I were mixed and left at room temperature for 15 minutes. After addition of 2 ml of LB medium and incubation at 30° C. for 2 hours, this cell suspension was spread on dishes of LB media containing 20 μg/ml of Km and 25 μg/ml of Cm, and then indicated at 30°C. for 24 hours. The 20 clones which appeared on the dishes were harvested and plasmid extraction was carried out by the mini-preparation method (Birnboim and Doly, Nucleic Acids Res., 7, 1513, 1979). Analysis of the restriction map showed that all the clones exhibited a plasmid in which MudII1734 was inserted. One of the m was designated pCE1134 (FIG. 4).

Construction of MudAM9 comprising the Thr operon

pCE1134 was completely cleaved by the BamHI and then SalI restriction enzymes. The DNA fragment in which the lac operon is lacking was isolated by agarose gel electrophoresis and extracted from this gel.

The pAJ294 plasmid (Miwa et al., Agric. Biol. Chem., 47, 2329, 1983) was completely cleaved by the BamHI and then SalI restriction enzymes. After agarose gel electrophoresis, the DNA fragment containing the thr operon was extracted from the gel. The two DNA fragments were mixed and ligation was carried out at 10° C. for 16 hours using T4-DNA ligase, ATP and dithiothreitol. After transformation of the thrB mutant strain with the ligation mixture, the transformants were selected on a minimum medium containing 25 μg/ml of Cm and no threonine. The plasmid was extracted from one of the transformants and was found to contain Mud with the npt gene and the thr operon (pAM9, FIG. 5).

Transposition of MudAM9 on the chromosomal DNA

The E. coli strain JM109 (Yanisch-Perron et al., Gene, 33, 103-119 (1985)) (Mucts) was transformed with pAM9 after treatment with calcium chloride. The transformants were selected on dishes of LB medium containing 20 μg/ml of Km and 25 μg/ml of Cm, after incubation at 30° C. for 24 hours. The plasmids were extracted from 20 independent colonies and were found to be pAM9. JM109 (Mucts)/pAM9 was cultured at 30° C. for 2 hours after being subjected to thermal shock at 45° C. for 15 minutes. The culture was maintained at 37° C. for 2 hours. The culture was then mixed with a few drops of chloroform and left at room temperature for 15 minutes. The phage solution II was obtained at 10 minutes of centrifugation at 5,000 rpm.

To the culture of JM109 (Mucts) left overnight at 30° C. in LB medium there were added CaCl₂ and MgSO₄, giving a final concentration of 1 mM and 2.5 mM respectively. 200 μl of this solution and of phage solution II were mixed and left at room temperature for 15 minutes. After addition of 2 ml of LB medium, incubation was carried out at 30° C. for 2 hours, and this cell suspension was spread on dishes of LB medium containing 20 μg/ml of Km. After incubation at 30° C. for 24 hours, 100 colonies were harvested and their sensitivity to Cm was checked. 95% of them were sensitive to Cm and were confirmed as JM109 (Mucts) (MudAM9).

After culturing JM109 (Mucts) (MudAM9) at 30° C. for 2 hours in LB medium, thermal shock treatment was carried out for 15 minutes at 45° C. followed by incubation at 37° C. for 2 hours. A few drops of chloroform were then added and centrifugation was carried out at 5,000 rpm for 10 minutes. The supernatant obtained was phage solution III.

To the E. coli strain AJ11332 (Shiio et al., Agric. Biol. Chem., 33, 1152, 1969), cultured overnight, there were added CaCl₂ and MgSO₄, giving final concentrations of 1 mMand 2.5 mM respectively. 200 μl of this solution and the phage solution III were mixed and left at room temperature for 15 minutes. After addition of 2 ml of LB medium, incubation was continued at 30° C. for 2 hours. The cell suspension was spread on dishes of LB medium containing 20 μg/ml of Km and then incubated at 30° C. for 24 hours. 100 colonies were harvested and tested for their lysis at 45° C. and their sensitivity to the Mu phage. One of these strains was designated EL1001.

Amplification of MudAM9 on the chromosomal DNA and choice of the copy number

(i) Amplification of MudAM9 by pPR30:

Strain EL1001 was transformed with pPR30. The transformants were selected on dishes of LB medium containing 25 μg/ml of Ap and a variable quantity of Km and/or of neomycin (Nm).

(ii) Amplification by lysate of Mucts:

Strain EL1001 was infected by a lysate of Mucts, such as described hereinabove, and then spread on dishes of LB medium containing variable quantities of Km.

The clones obtained from pPR30 in the case of (i) and those obtained from Mu in the case of (ii) were selected in order to provide a stable number of copies of MudAM9.

After incubation at 30° C. for 48 hours, the colonies were harvested and the number of MudAM9 on the chromosomal DNA was measured by the hybridization method described hereinabove.

Strains EL1002 and EL1003 will be studied more particularly.

The DNA was extracted from strains AJ11332, EL1001, EL1002 and EL1003. After digestion by the EcoRI restriction enzyme, the DNAwas subjected to electrophoresis and transferred to a nylon membrane. The DNA was then hybridized with small radiolabeled pAM9 fragments digested by EcoRI. The results are shown in FIG. 6. Bands A, B, C and D correspond respectively to the strains AJ11332, EL1001, EL1002 and EL1003. The letter a corresponds to the wild type of the thr operon, and the letter b corresponds to an unidentified band seen in lanes B, C and D. The numbers correspond to MudAM9-specific bands. According to these results strains EL1001, EL1002 and EL1003 have 1, 4 and 10 copies (at least) of MudAM9.

The relationship between the copy number of MudAM9 in the strain and the concentration of antibiotics to which the strain is resistant is summarized in Table I:

                  TABLE I                                                          ______________________________________                                         Relationship between the copy number of MudAM9                                 and antibiotic resistance                                                      No. of copies of MudAM9                                                                       Resistance   Sensitivity                                        ______________________________________                                         0                            20 μg/ml KM                                    1              200 μg/ml Km                                                                             250 μg/ml of                                                                Km and Nm                                          2 (or more)    250 μg/m of                                                                 Km and Nm                                                       ______________________________________                                    

Production of L-threonine

The strains having amplified MudAM9 were refreshed on dishes of LB medium at 30° C. for 24 hours and inoculated in 20 ml of the production medium described hereinbelow.

Composition of the production medium

Glucose: 30 g/l, (NH₄)₂ SO₄ : 10 g/l, KH₂ PO₄ : 1 g/l, MgSO₄ 7H₂ O; 1 g/l, FeSO₄ 7H₂ O: 10 mg/l, MnSO₄ 4-6H₂ O: 10 mg/l, L-Met: 0.1 g/l, L-Ile: 0.1 g/l, L-Pro: 0.45 g/l, RNA: 2 g/l, thiamine HCl: 1 mg/l and CaCO₃ : 40 g/l (pH: 7.0).

The culture was stirred at 30° C. for 72 hours. The quantity of threonine in the medium was measured with an amino acid analyzer. The results are shown in Table II:

                  TABLE II                                                         ______________________________________                                         Production of L-threonine by strains containing MudAM9                                    Estimation of the copy                                              Strain     number of MudAM9                                                                              L-Thr (g/l)                                          ______________________________________                                         AJ11332     0             3                                                    EL1001      1 (at least)  3.3                                                  EL1002      4 (at least)  8.9                                                  EL1003     10 (at least)  12                                                   ______________________________________                                    

Strain EL1002 containing 4 copies of MudAM9 was transferred on several successive occasions onto dishes of LB medium without antibiotics, and after each transfer the L-threonine productivity was tested by using the method described given above. The results are shown in Table III:

                  TABLE III                                                        ______________________________________                                         Stability of L-threonine production after repeated                             transfers on LB media without antibiotics with                                 strain EL1002                                                                  ______________________________________                                         Number of transfers                                                                           0     2       4   6     8   10                                  L-Thr production                                                                              8.5   8.6     8.5 8.8   8.8 8.9                                 ______________________________________                                    

Table IV shows the specific activities of the L-Thr biosynthesis enzymes of strains containing MudAM9:

                  TABLE IV                                                         ______________________________________                                         High-quantity production of L-threonine biosynthesis                           enzymes by AJ11332 (mudAM9)                                                                 Specific activities (*)                                           Strain         AK1      HDHI                                                   ______________________________________                                         AJ11332        2.5      4.8                                                    EL1001         4.0      10.1                                                   EL1002         12.5     25.0                                                   EL1003         22.5     37.4                                                   ______________________________________                                          (*) The specific activities are given in μmole/min/mg of protein      

EXAMPLE 3: AMPLIFICATION OF MUD WITH A PLASMID CONTAINING THERMOINDUCTIBLE TRANSPOSASES

Construction of a plasmid containing thermoinductible transposases

The strain POII1681 : M8820 (Mucts) (MudII1681) (Castilho et al., J., Bacteriol., 158, 488 (1984)) was transformed, after treating the bacteria with CaCl₂ (Mandel M. and Higa A., J. Mol. Biol., 53, 159 (1970)), with the pEV11 plasmid (FIG. 7). After 24 hours of incubation at 30° C. the transformants were selected on dishes of LB medium containing 20 μg/ml of Ap and 20 μg/ml of Km. A transformant POII1681/pEV11 was cultured for 2 hours at 30° C. in 3 ml of LB. The culture was then subjected to thermal shock for 15 minutes at 45° C. followed by culturing at 37° C. for 2 hours. A few drops of chloroform were added, and after centrifugation at 5,000 rpm for 10 minutes the phage solution was obtained. The strain E. coli : M8820 (Mu) (Castilho etal., J., Bacteriol., 158, 488 (1984)) was cultured overnight in LB medium; CaCl₂ and MgSO₄ were added, giving final concentrations of 1 mM and 2.5 mM respectively. To 200 μl of this culture 50 μl of the phage solution were added, this suspension being left at room temperature for 15 minutes.

After adding 2 ml of LB and stirring for 2 hours at 30° C., the suspension of bacteria was spread on dishes of LB medium containing 20 μg/ml of Ap and 20 μg/ml of Km; incubation was carried out for 24 hours at 30° C. 20 colonies were obtained and their plasmid DNAs were extracted by the mini-preparation method (Birnboim and Doly, Nucleic Acid Res. 7, 1513 (1979)). After analyses of the restriction map, the clones were found to contain an insertion of the MudII1681 in the pEVll plasmid. One of the plasmids is designated pMC1 (FIG. 7).

The pMC1 plasmid was digested by the BamHI and BglII restriction enzymes and then ligated with pRC100 (see Example 2) cut by BamHI. The strain MC1060 was transformed by the ligation mixture. The transformants were selected on dishes of LB medium containing 25 μg/ml of Cm and 20 μg/ml of Ap. The plasmids of 30 transformants were extracted and analyzed; one of them was designated pMC23 (FIG. 7).

Amplification of the defective MudAM9 phage with the pMC23 plasmid

The strain EL1001 containing the defective MudAM9 phage (see Example 2) was transformed, after treatment with CaCl₂ (Example 2), by the pMC23 plasmid. The transformants were selected on dishes of LB medium containing 20 μg/ml of Ap and 25 μg/ml of Cm after an incubation of 24 hours at 30° C.

A transformant EL1001/pMC23 was cultured in LB medium containing 20 μg/ml of Ap and 25 μg/ml of Cm for 2 hours at 30° C. 1 ml of this culture was subjected to a thermal shock for 15 minutes at 45° C., then 1 ml of LB was added; the incubation took place at 30° C. for 2 hours.

This bacterial suspension was spread on dishes of LB medium containing 400 μg/ml of Nm and Km; incubation was carried out at 30° C. for 48 hours. 50 clones appeared and were tested for their sensitivity to Can and their L-Thr production (Example 2). 50% of them were Cm-sensitive and 60% produced more L-threonine than the host strain.

Several clones were thus selected, and their copy number of MudAM9 was measured by the DNA-DNA hybridization technique (Southern, 1975, J. Mol. Biol., 98, 503). The chromosomal DNAs were digested by the HindIII restriction enzyme. After electrophoresis, the DNAs were absorbed on a nylon membrane and then hybridized with the SalI-BamHI fragment (labeled by sulfonation), containing the threonine operon, of the pAM9 plasmid as the probe.

The results relating to the production of L-threonine and the copy number of MudAM9 are given in Table V and FIG. 8. Lanes A, B and C correspond respectively to the strains AJ11332, EL1008 and EL1009.

In the A lane the "o" band corresponding to the threonine operon is shown. In lane B at least 6 bands are shown: band "o" and 5 bands which are numbered 1 to 5 and which correspond to 5 copies (at least) of MudAM9. In lane C, in addition to the "o" band, 3 bands are shown numbered 1 to 3 and corresponding to 3 copies at least of MudAM9.

                  TABLE V                                                          ______________________________________                                         Copy number of MudAM9 and production of                                        L-threonine in strains EL1008 and EL1009                                       Strain        Copy number                                                                               L-Thr (g/l)                                           ______________________________________                                         EL1008        5          7                                                     EL1009        3          5                                                     ______________________________________                                    

EXAMPLE 4: AMPLIFICATION OF TWO DIFFERENT DEFECTIVE PHAGES ON THE CHROMOSOMAL DNA OF A STRAIN

Preparation of a lysate containing MudIIPR13

After culturing the strain MC4100 (Mucts) (MudII-PR13) (P. Rarer et al., Gene, 63, 41-52, 1988) in LB medium for 2 hours at 30° C., a thermal shock was carried out for 15 min at 45° C., followed by incubation at 37° C. for 2 hours. A few drops of chloroform were added, and then centrifugation was carried out at 5,000 rpm for 10 minutes. The phage lysate VI was thus obtained.

Transposition of MudIIPR13 on the chromosomal DNA

To an overnight culture of strain EL1001, CaCl₂ and MgSO₄ were added, giving final concentrations of 1 mM and 2.5 mM respectively. 50 μl of the phage solution VI were added to 200 μl of the EL1001 culture, and the suspension was then left to stand at room temperature for 15 min. After addition of 2 ml of LB, the suspension was stirred at 30° C. for 2 hours. The bacteria were then spread on an LB medium containing 25 μg/ml of Cm and 20 μg/ml of Km. After incubation for 24 hours at 30° C., 50 colonies were subcultured and then tested for their lysis at 45° C. and for their sensitivity to the Mu phage. 80% of the colonies tested were sensitive to Mu and did not lyse at 45° C. One EL1001 (MudIIPR13) strain was designated EL1004.

The strain EL1003 was treated in the same manner.

One EL1003 (MudIIPR13) strain was designated EL1005.

Amplification of MudIIPR13 in the EL1004 and EL1005 strains

The EL1004 strain was infected by a Mucts lysate prepared from the JM109 (Mucts) strain by the method described in Example 2. The bacterial suspension was spread on dishes of LB medium containing 20 μg/ml of Km and 400 μg/ml of Cm. After incubation for 48 hours at 30° C., 50 colonies were isolated and then tested for their lysis at 45° C. and for their sensitivity to the Mu phage. 84% of the clones were sensitive to Mu and did not lyse at 45° C. One of these clones was designated EL1006.

The EL1005 strain was treated in the same manner and the EL1007 clone was obtained.

The chromosomal DNAs were extracted from strains AJ11332, EL1001, EL1003, EL1004, EL1005, EL1006 and EL1007.

After digestion of the chromosomal DNAs by the EcoRI restriction enzyme, a DNA-DNA hybridization was carried out by the Southern method (Example 3). The probe corresponded to the pRC100 plasmid digested by HindIII and labeled by sulfonation. In FIG. 9 the lanes A, B, C, D, E, F and G correspond respectively to strains AJl1332, EL1001, EL1004, EL1006, EL1003, EL1005 and EL1007.

The bands obtained in the lanes A, B and E correspond to the hybridization between the pRC100 probe and the chromosomal DNAs of the strains. These bands are present in all the other lanes and are not therefore specific to MudIIPR13. In lanes C, D, F and G the numbers 1 to 4 mark the bands having a specificity for hybridization with MudIIPR13, each band corresponding to an insertion of the MudIIPR13. In these same lanes the "o" band corresponds to an internal band of the MudIIPR-13. According to these results strains EL1004 and EL1005 contain 1 insertion of the MudIIPR13; strains EL1006 and EL1007 contain, respectively, 4 and 3 insertions of MudIIPR13 at least.

EXAMPLE 5: INTEGRATION AND AMPLIFICATION OF DEFECTIVE PHAGES IN AN ERWINIA CHRYSANTHEMI STRAIN

Preparation of lysates of MudII1734 and MudAM9

Two phage lysates containing MudII1734 and MudAM9 were prepared from strains: POII1734 (Castilho et al., J. Bacteriol., 158, 488 (1984)) and JM109 (Mucts) (MudAM9) (Example 2) according to the method described in Example 2.

Transposition of the Mud phages on the chromosomal DNA of Erwinia chrysanthemi

The E. chrysanthemi strain 384551 (M. Chippaux, CNRS, Marseilles) was used as the recipient strain. The transduction (Example 2) was carried out using 2 lysates (Example 2). The colonies capable of growing in a medium containing 20 μg/ml of Fun were isolated and then tested for their lysis at 45° C. and their sensitivity to the Mu phage. 70% and 80% respectively of the transductants obtained after infection by the MudII1734 and MudAM9 lysates were sensitive to the Mu phage and did not lyse at 45° C. One clone from each transduction was selected:

EL3003: E. chrysanthemi (MudII1734)

EL3004: E. chrysanthemi (MudAM9)

The amplification of the Mud phages in strains EL3003 and EL3004 was carried out using a lysate of strain JM109 (Mucts) as described in Example 2. The selection of the strains containing the amplified Mud was effected in an LB medium containing 400 μg/ml of Km and Nm.

50 resistant colonies were subcultured and then tested for their Mu phage-sensitivity. Strains EL3007 and EL3008 sensitive to the Mu phage were thus obtained.

Copy number of Mud on the chromosomal DNA of the two strains

The chromosomal DNA was extracted by the same technique as for E. coli (Example 1) except for a treatment with SDS in place of sarcosyl. Molecular analysis of these DNAs was carried out by the Southern method (Example 3). The DNAs were digested by the EcoRI restriction enzyme. The probes used were specific to the defective phages and were labeled by sulfonation: For MudII1734: EcoRI-HindIII fragment of the pAM9 plasmid corresponding to the left border of the defective phage. For MudAM9: SalI-BamHI fragment of the pAM9 plasmid corresponding to the threonine operon of E. coli.

The results are shown in Table VI and FIGS. 10 and 11.

In FIG. 10 the lanes A, B and C correspond respectively to the strains: E. chrysanthemi 384551, EL3003 and EL3007.

In lanes B and C, 1 and 3 bands appear corresponding to a hybridization specificity with MudII1734. According to these results strains EL3003 and EL3007 possess, respectively, 1 and 3 insertions (at least) of MudII1734.

In FIG. 11 lanes A, B and C correspond respectively to the strains: E. chryasanthemi 384551, EL3004 and EL3008.

In lane A the "b" fragment corresponds to a hybridization between the probe and the chromosomal DNA of the 384551 strain. This fragment is also found in all the other lanes.

In lanes B and C the numbers correspond to bands having a specificity of hybridization with MudAM9. Two bands correspond to one insertion of MudAM9.

According to these results strains EL3004 and EL3008 possess, respectively, 1 and 4 insertions at least of MudAM9.

                  TABLE VI                                                         ______________________________________                                         Copy number of Mud in strains of E. chrysanthemi                               Strain              Copy number                                                ______________________________________                                         EL3003              1                                                          EL3004              1                                                          EL3007              3 (at least)                                               EL3008              4 (at least)                                               ______________________________________                                    

EXAMPLE 6: AMPLIFICATION OF A DEFECTIVE PHAGE OF LARGER SIZE

Construction of the MudAM11 comprising the threonine operon and the asd gene

The pAM9 plasmid (see Example 2), containing the defective MudAM9 phage, is completely cleaved by the BamHI restriction enzyme (single site); pAM9 is thus linearized.

The pAD20 plasmid (Haziza et al., EMBO, J., 1982, 3, 379-384) is completely cleaved by the BamHI restriction enzyme. After electrophoresis, the fragment containing the asd gene is extracted from the gel.

The two DNA fragments are mixed and ligation is carried out at 22° C. for 1 hour, using T4-DNA ligase, ATP and dithiothreitol. After transformation of a mutant strain Asd⁻ : JCP203 (C. PRINTZ, personal communication) with the ligation mixture, the transformants were selected in a complete medium containing 25 μg/ml of Cm and 20 μg/ml of Km. A plasmid extracted from the transformants was found to have the defective MudAMll phage containing the nptII gene, the threonine operon and the asd gene. This ptasmid designated pAMll is shown in FIG. 12.

Transposition of MudAMll on the chromosomal DNAs

The strain JM109 (Mucts)/pAMll was obtained by transformation of the strain of E. coli JM109 (Mucts) (see Example 2, page 14, lines 5 to 11), by the pAMll plasmid.

The strain JM109 (Mucts)/pAMll was cultured at 30° C. for 2 hours. After being subjected to thermal shock at 45° C. for 15 minutes, the cultures were maintained at 37° C. for 2 hours. A few drops of chloroform were then added to this culture and it was left at room temperature for 15 minutes. The phage solution II-AMll was obtained after 10 minutes of centrifugation at 5,000 rpm.

To the culture of JM109 (Mucts) left overnight at 30° C. in LB medium there were added CaCl₂ and MgSO₄ giving final concentrations of 1 mM and 2.5 mM, respectively. 50 μl of the phage solution II-AMll were mixed with 200 μl of this culture and left at room temperature for 15 minutes. After addition of 2 ml of LB medium, the mixture was stirred for 2 hours at 30° C. and the bacterial suspension was then spread on dishes of LB medium containing 20 μg/ml of Km. After incubation at 30° C. for 24 hours, 100 colonies were harvested and tested for their sensitivity to Cm, 85% of them showing such sensitivity, and one clone being designated JM109 (Mucts) (MudAM11).

After culturing strain JM109 (Mucts) (MudAM11) for 2 hours at 30° C. in LB medium, a thermal shock treatment was carried out for 15 minutes at 45° C. After incubation for 2 hours at 37° C., a few drops of chloroform were added and centrifugation was carried out at 5,000 rpm for 10 minutes.

The supernatant obtained was the phage solution III-AM11.

The strain AJl1332 (λ⁺, Pro⁻) was transduced by a P1 phage cultured in a Pro⁺ strain. A strain AJl1332 (λ⁺, Pro⁺) was selected. The λ phage contained in this strain was then eliminated (C. MOREL, personal communication). The strain thus obtained: AJ11332 (λ⁻, Pro⁺) was designated EL1016. To the strain of E. coli EL1016, cultured overnight at 37° C. in LB, there were added CaCl₂ and MgSO₄, giving final concentrations of 1 mM and 2.5 mM respectively. 50 μl of the phage solution III-AM11 were mixed with 200 μl of this culture and left at room temperature for 15 minutes. After addition of 2 ml of LB, stirring was carried out for 2 hours at 30° C. The bacterial suspension was spread on dishes of LB medium containing 20 μg/ml of Km, then incubation was carried out for 24 hours at 30° C. A Mu-sensitive clone was selected=EL1012.

Amplification of MudAMll on chromosomal DNAs and choice of copy number

The amplification was effected by infection of the EL1012 strain by a Mucts phage lysate obtained as above. The bacteria were spread on dishes of LB medium containing variable quantities of Km and Nm ranging from 250 to 1200 μg/ml.

The selected transductants contained a stable number of copies of MudAM11.

The strain EL1013 was studied more particularly.

The MudAM11 number on the chromosomal DNAs was evaluated by the Southern DNA-DNA hybridization method described in Example 3.

The chromosomal DNAs were extracted from strains EL1016, EL1012 and EL1013 respectively. After digestion by the SalI or EcoRI restriction enzymes, these DNAs were subjected to electrophoresis and were then absorbed on a nylon membrane (see Example 3). They were then hybridized with a DNA fragment labeled (by sulfonation) as the probe (FIGS. 13, 14, 15 and 16), lanes A, B and C corresponding respectively to strains EL1016, EL1012 and EL1013.

The letter "a" corresponds to the wild type of the threonine operon or the asd gene. The letter "p" corresponds to partial digestions of the DNAs. The numbers correspond to the bands marking a specificity of hybridization with MudAM11.

In FIG. 13 the chromosomal DNAs were digested by SalI and hybridized with the EcoRI-HindIII fragment containing the nptII, thrA, thrB and thrC' genes, of pAM9 as the probe.

Two bands correspond in this case to one insertion of MudAM11.

In lane A only the "a" band corresponding to the original threonine operon is shown.

In lane B three bands are shown : "a" and "1" and "2" which correspond to an insertion of MudAM11.

In lane C nine bands are shown : "a" and "1" to "8" which correspond to 4 insertions (at least) of MudAM11.

In FIG. 14 the chromosomal DNAs were digested by EcoRI and hybridized with the EcoRI-HindIII fragment containing the nptII, thrA, thrB and thrC' genes of pAM9 as the probe. One band corresponds to one insertion of MudAM11.

In lane A only the "a" band corresponding to the threonine operon is shown.

The "p" band corresponds to a partial digestion. In lane B two bands are shown. "a" and "1" corresponding to an insertion of MudAM11.

The "p" band corresponds to a partial digestion.

In lane C six bands are shown: "a" and "1" to "5" corresponding to 5 insertions (at least) of MudAM11.

In FIG. 15 the chromosomal DNAs were digested by SalI and hybridized with the BamHI fragment, containing the asd gene, of pAM11 as the probe.

One band thus corresponds to one insertion of MudAM11.

In lane A only the "a" band is shown, corresponding to the asd gene.

In lane B two bands are shown: "a" and "1", that is one insertion of MudAM11.

In lane C five bands are shown: "a" and "1" to "4" which correspond to 4 insertions (at least) of MudAM11.

In FIG. 16 the chromosomal DNAs were digested by EcoRI and hybridized with the BamHI fragment, containing the asd gene, of pAM11 as the probe.

One band corresponds to one insertion of MudAM11.

In lane A only the "a" band corresponding to the asd gene is shown.

In lane B two bands are shown: "a" and "1", that is one insertion of MudAM11.

In lane C six bands are shown: "a" and "1" to "5", that is 5 insertions (at least) of MudAM11.

According to these results strains EL1012 and EL1013 are, respectively, one and five copies (at least) of MudAM11.

                  TABLE VII                                                        ______________________________________                                         Relationship between the copy number of MudAM11                                in the strains and the antibiotic concentrations                               to which they are resistant                                                    Copy number MudAM11                                                                            Resistance  Sensitivity                                        ______________________________________                                         0                            20 μg/ml Km                                    1               200 μg/ml                                                                                250 μg/ml                                                      Nm and Km   Nm and Km                                          5 (or more)     250 μg/ml                                                                               1200 μg/ml                                                      Nm and Km   Nm and Km                                          ______________________________________                                    

                  TABLE VIII                                                       ______________________________________                                         Production of L-threonine by strains EL1012                                    and EL1013 containing the defective MudAM11 phage                              The strains having amplified MudAM11 were sub-                                 cultured in dishes of LB medium at 30° C. for 24 hours and              inoculated in 20 ml of the production medium (Example 2).                      Culturing was carried out at 30° C. for 72 hours. The                   quantity of L-threonine in the medium was measured using                       an amino acid analyzer.                                                                   Estimation of the copy                                              Strains    number of MudAM11                                                                             L-Thr (g/l)                                          ______________________________________                                         EL1016     0              3.0                                                  EL1012     1              5.3                                                  EL1013     5 (at least)   10.2                                                 ______________________________________                                    

                  TABLE IX                                                         ______________________________________                                         Specific activities of the L-threonine biosynthesis                            enzymes of the strains containing MudAM11                                      The aspartokinase-I (AK-I) activity was assayed                                by the method of Truffa-Bachi, P., Cohen, G-N., Methods                        Enzymology, 1970, 17, 694-702.                                                 The aspartate semialdehyde dehydrogenase (ASA)                                 activity was measured using the technique of Hegeman, G-                       D., Cohen, G-N., Margan, R., Methods Enzymology, 1970,                         17, 708-713.                                                                              Specific activities                                                            μ/mole/min/mg of proteins                                        Strains      AK-1       ASA                                                    ______________________________________                                         EL1016       4.9        0.32                                                   EL1012       4.0        0.54                                                   EL1013       21.1       1.14                                                   ______________________________________                                    

EXAMPLE 7: AMPLIFICATION OF THE aspA GENE OF E. COLI WITH A VIEW TO THE PRODUCTION OF L-ASPARTATE AMMONIA-LYASE

Construction of MudAB9 comprising the aspA gene coding for the L-aspartate ammonia-lyase of E. coli

The pGS94 plasmid (Guest et al., J. Gen. Microbiol. (1984), 130, 1271-1278) containing the aspA gene is completely cleaved by the ClaI and SalI restriction enzymes. The 3.4 Kb fragment contains the aspA gene.

The pPC3 plasmid (resulting from inversion of PstI fragment of pPR3 plasmid; GENE, Vol. 42, (1986) pages 185-192) containing the defective MudPC3 phage is completely cleaved by the ClaI and SalI restriction enzymes. The 5.3 Kb fragment contains the defective phage in which it is desired to clone the aspA gene.

The two cleaved plasmids are mixed and ligation is carried out at 22° C. for 1 hour, using T4-DNA ligase, ATP and dithiothreitol. After transformation (Example 2) of the MC1060 strain with the ligation mixture, the transformants are selected on LB medium containing 20 μg/ml of Ap and 25 μg/ml of Cm.

27 transformants were subcultured on LB medium containing 20 μg/ml of Km; 17 clones sensitive to this antibiotic were retained.

The L-aspartate ammonia-lyase activity was assayed using acellular extracts of cultures of these clones according to the method of Spencer et al., J. Gen. Microbiol. (1976), 97, 73-82. Only clone 9 had an AspA activity higher than the activity conferred by the chromosomal gene of the MC1060 strain.

The plasmid DNAof this clone was extracted using the Birboim and Doly technique and then cleaved by the EcoRI and HindIII restriction enzymes. In parallel, the DNAs of the two parent plasmids were cleaved by the same restriction enzymes. Analysis of the restriction profiles showed that clone 9 corresponds to the cloning of the fragment containing the aspagene in the defective MudPC3 phage. The new defective phage was designated MudAB9 and the plasmid carrying it pAB9 (FIG. 17).

Transposition of MudAB9 on chromosomal DNAs

The strain JM109 (Mucts)/pAB9 was obtained by transformation of the strain E. coli JM109 (Mucts) (Example 2) by plasmid pAB9.

The strain JM109 (Mucts)/pAB9 was cultured at 30° C. for 2 hours. After being subjected to thermal shock at 45° C. for 15 minutes, the culture was maintained at 37° C. for 2 hours. A few drops of chloroform were then added; after centrifugation at 5,000 rpm for 10 minutes, the phage lysate II-AB9 was thus obtained.

The strain JM109 (Mucts) was cultured overnight at 30° C. in LB medium. CaCl₂ and MgSO₄ were then added, giving concentrations of 1 mMand 2.5 mM respectively. To 200 μl of this culture 50 μl of phage lysate II-AB9 were added. After 15 minutes of adsorption at room temperature, 2 ml of LB were added and the tubes were stirred at 30° C. for 2 hours. The suspension was then spread on dishes of LB medium containing 25 μg/ml of Cm.

The dishes were placed in an oven at 30° C. for 24 hours. 100 colonies were picked up and checked for their sensitivity to ampicillin. Strain JM109 (Mucts) (MudAB9) was thus obtained. The phage lysate III-AB9 was obtained from this strain as previously described.

To the strain E. coli MC41000, cultured overnight at 37° C. in LB medium, CaCl₂ and MgSO₄ were added to give concentrations of 1 mM and 2.5mM respectively. The phage solution III-AB9 was mixed with 200 μl of this culture and left at room temperature for 15 minutes. After addition of 2 ml of LB, incubation was continued at 30° C. for 2 hours. The cell suspension was spread on a dish of LB medium containing 25 μg/ml of Cm. The dish was then placed in an oven at 30° C. for 24 hours. An Mu-sensitive clone was selected: EL1010.

Amplification of MudAB9 on chromosomal DNAs

Determination of copy number

Amplification was carried out by infection of the EL1010 strain by a Mucts lysate obtained as described hereinabove. The bacteria were spread on dishes of LB medium containing 500 μg/ml of Cm.

The selected transductants contained a stable copy number of MudAB9.

After incubation at 30° C. for 48 hours, the resistant colonies were picked up and the copy number of defective MudAB9 phages was determined by the Southern DNA-hybridization method (Example 3).

Strains EL1010 and EL1011 were studied in particular.

The DNA was extracted from strains MC4100, EL1010 and EL1011. After digestion by the SalI restriction enzyme, the DNA was subjected to electrophoresis and then absorbed on a nylon membrane. The DNA was subsequently hybridized with a plasmid labeled by sulfonation containing the aspA gene.

In FIG. 18 lanes A, B and C correspond respectively to strains MC4100, EL1010 and EL1011. The letter "a" corresponds to the wild type of the aspA gene.

In lane A, only the "a" band corresponding to the aspA gene is shown.

In lane B two bands are shown: "a" corresponding to the asDA gene, and "1" corresponding to an insertion of MudAB9.

In lane C five bands (at least) are shown: the "a" band and the "1" to "4" bands corresponding to 4 insertions (at least) of MudAB9.

                  TABLE X                                                          ______________________________________                                         Relationship between the copy number of MudAB9                                 in the strains, and their specific L-aspartate                                 ammonia-lyase activity                                                                    Copy number                                                                               Specific activity                                        Strains    of MudAB9  Δ DO/mn/mg Prot.                                   ______________________________________                                         MC4100     0          0.33                                                     EL1010     1          0.60                                                     EL1011     4 (at least)                                                                              2.04                                                     ______________________________________                                    

EXAMPLE 8: SEVERAL INSERTIONS IN ONE LYSOGENIZATION STAGE

A phage lysate was obtained from strain JM109 (Mucts) (MudAM9) as described in Example 2.

To an overnight culture of the strain E. coli: EL 1016 (Example 6), CaCl₂ and MgSO₄ were added to give final concentrations of 1 mM and 2.5 mM respectively. To 200 μl of this culture 50 μl of phage solution III-AM9 were added (Example 2) and this suspension was left at room temperature for 15 minutes.

After addition of 2 ml of LB, incubation was carried out for 2 hours at 30° C. The bacterial suspension was spread on dishes of LB medium containing 250 μg/ml of Km and Nm; incubation was carried out for 24 hours at 30° C. Two colonies appeared and were tested for their lysis at 45° C., and for their sensitivity to the Mu phage. These two clones did not lyse, were sensitive to Mu and were designated EL1014 and EL1015.

An L-threonine production test was carried out as described in Example 2. The copy number of MudAM9 was measured by the Southern DNA-DNA hybridization method (Example 3). The chromosomal DNAs were digested by the HindIII restriction enzyme. After electrophoresis, the DNAs were absorbed by a nylon membrane and then hybridized with the SalI-BamHI fragment (labeled by sulfonation), containing the threonine operon, of the pAM9 plasmid. The results concerning L-threonine production and the copy number of MudAM9 are given in Table XI and FIG. 19.

In FIG. 19 lanes A, B and C correspond respectively to strains EL1016, EL1014 and EL1015. The letter "o" corresponds to the wild type of the threonine operon. Each of the other bands corresponds to one insertion of MudAM9.

In lane A only the "o" band corresponding to the threonine operon is shown. In lane B, 3 additional bands are shown numbered 1 to 3 and corresponding to at least 3 insertions of MudAM9. In lane C, 2 additional bands correspond to 2 insertions of MudAM9 at least.

                  TABLE XI                                                         ______________________________________                                         Copy number and production of L-threonine                                      Strain    Copy number                                                                               Production of L-Thr (g/l)                                 ______________________________________                                         EL1014    3          6.2                                                       EL1015    2          5.3                                                       ______________________________________                                    

EXAMPLE. 9: STABILITY OF THE LOCALIZATION AND NUMBER OF DEFECTIVE PHAGES DURING FERMENTATION

The strain EL1003 (λ⁺, Pro⁻) was transduced by a P1 phage cultured on a Pro⁺ strain. A strain EL1003 (λ⁺, Pro⁺) was selected. The λ phage contained in this strain was then eliminated (C. Morel, personal communication). The strain thus obtained: EL1003 (λ⁻, Pro⁺ was designated EL1017.

The EL1017 strain containing 10 copies (at least) of MudAM9 was tested before and after fermentation in order to determine the stability of MudAM9 during this stage.

The fermenter containing 1.5 liters of fermentation medium (Example 2: components of the medium) was cultured with the EL1017 strain; the fermentation took place for 48 hours at the appropriate temperature. Clones were isolated after fermentation, and 4 clones were studied in particular: EL1017-a, EL1017-b, EL1017-c and EL1017-d.

The parent strain EL1017 and also the clones EL1017-a, EL1017-b, EL1017-c and EL1017-d were analyzed by the Southern DNA-DNA hybridization technique (Example 3) by which means it was possible to define the localization and the number of defective phages in the clones tested.

The chromosomal DNAs of strains EL1016 (Example 6), EL1017, EL1017-a, EL1017-b, EL1017-c and EL1017-d were extracted and then digested by the HindIII restriction enzyme.

After electrophoresis, they were absorbed on nylon membrane and hybridized with the EcoRI-HindIII fragment labeled (by sulfonation) (containing the genes nptII, thrA, thrB and thrC') of the pAM9 plasmid (FIG. 10). The result of the DNA-DNA hybridization is shown in FIG. 20.

Lanes A, B, C, D, E and F correspond respectively to strains EL1016, EL1017, EL1017-a, EL1017-b, EL1017-c and EL1017-d.

The letter "a" corresponds to the wild type of the threonine operon.

The numbers correspond to the bands marking the specificity of hybridization with MudAM9. One band corresponds to one insertion of MudAM9.

Lane A=Only the "a" band corresponding to the threonine operon is shown.

Lanes B, C, D, E and F: 10 bands (at least) are shown, the "a" band and 9 others numbered "1" to "9" corresponding to 9 insertions (at least) of MudAM9.

According to these results the hybridization profiles of the parent strain and the clones resulting from fermentation are similar. This reveals the stability of the defective phages inserted in the chromosomes of the bacteria used for the fermentation.

The following strains were deposited at the Collection Nationale de Cultures de Microorganismes, Institut Pasteur, 25 rue du Docteur-Roux, Paris (15th) on Jan. 25, 1988:

    ______________________________________                                         Escherichia coli K12 POII1681                                                                        under No. I-727                                          Escherichia coli K12 POII1734                                                                        under No. I-728                                          Escherichia coli K12 EL1001                                                                          under No. I-729                                          Escherichia coli K12 EL1002                                                                          under No. I-730                                          Escherichia coli K12 EL1003                                                                          under No. I-731.                                         ______________________________________                                     

We claim:
 1. A process for preparing an Escherichia coli or Erwinia chrysanthemi bacterial strain in which a specific number of copies of a chosen gene or of a chosen DNA molecule is integrated into a chromosome or episome of an Escherichia coli or Erwinia chrysanthemi bacterium wherein said process comprises:a) cloning said chosen gene or said chosen DNA molecule into a transposon, wherein said transposon is incapable of expressing a functional transposase, said transposon is from a Mu phage and at least one marker gene is situated within said transposon; b) integrating said transposon containing said chosen gene or said chosen DNA molecule into the chromosome or episome of said bacterium; c) complementing said transposon with a functional transposase gene; d) expressing said functional transposase gene, whereby transposition of said transposon occurs; e) selecting said bacterial strain on a medium that is selectable for said marker gene in a manner such that a bacterial strain containing a specific number of copies of said chosen gene or chosen DNA molecule is obtained; and f) recovering the bacterial strain thereby selected.
 2. The process as claimed in claim 1, wherein said transposon is contained on a plasmid and is introduced into said bacterium by transformation with said plasmid.
 3. The process as claimed in claim 1, wherein the said transposon is contained in a bacteriophage and the said transposon is introduced into the bacteria by transduction with the said bacteriophage.
 4. The process as claimed in claim 1, wherein said chosen gene is under the control of a promoter within said transposon, wherein said promoter directs the expression of said chosen gene in said bacterial strain.
 5. The process as claimed in claim 1, wherein said transposon is a MudI or MudII phage.
 6. The process as claimed in claim 1, wherein said marker gene is an antibiotic-resistant gene expressed in said bacterial strain.
 7. The process as claimed in claim 1 wherein the number of transpositions is assessed by determination of the expression of the marker gene.
 8. The process as claimed in claim 7, wherein said marker gene is an antibiotic-resistance gene and wherein expression of the marker gene is determined by the resistance of said bacterial strain to an antibiotic.
 9. The process as claimed in claim 7, wherein said marker gene is a β-galactosidase gene and wherein expression of the marker gene is determined by a change in color of a substrate of β-galactosidase.
 10. The process as claimed in claim 1 wherein the said chosen gene is one or more of the genes encoding proteins of the biosynthesis pathway of L-threonine.
 11. The process as claimed in claim 1 wherein the said chosen gene codes for a protein.
 12. A method for production of a protein product of a chosen gene, said method comprising:preparing a bacterial strain in which a specific number of copies of said chosen gene is integrated into a chromosome or episome according to the method of claim 1, and growing said bacterial strain in a suitable culture medium for sufficient time for said chosen gene to be expressed and said protein to be produced.
 13. A method according to claim 12, wherein said complementing is by transformation of said bacterial strain with a plasmid comprising a transposase or by transfection with a Mu phage; and which further comprises the step of:after a specific number of transpositions, selecting bacteria cured of said plasmid or said Mu phage.
 14. The process of claim 1, comprising the further steps of:g) cloning a second chosen gene or a second chosen DNA molecule into a second transposon, wherein said second transposon is incapable of expressing a functional transposase, said second transposon is a Mu phage and at least one second marker gene is situated within said second transposon; h) integrating said second transposon containing said second chosen gene or said second chosen DNA molecule into the chromosome or episome of said bacterium; i) complementing said second transposon with a functional transposase gene; j) expressing said functional transposase gene, whereby transposition of said second transposon occurs; k) selecting said bacterial strain on a medium that is selectable for said second marker gene in a manner such that a bacterial strain containing a specific number of copies of said second chosen gene or second chosen DNA molecule is obtained; and l) recovering the bacterial strain thereby selected.
 15. The process as claimed in claim 14, wherein said complementing is by transformation of said bacteria with a plasmid expressing the functional transposase or by transduction with a Mu phage.
 16. A process for preparing an Escherichia coli or Erwinia chrysanthemi bacterial strain in which a specific number of copies of a chosen gene or of a chosen DNA molecule is integrated into a chromosome or episome of a bacterium, wherein said process comprises:a) cloning said chosen gene or said chosen DNA molecule into a transposon, said transposon having a functional transposase gene the expression of which is repressed by a temperature sensitive Mu repressor, wherein said transposon is a Mu phage and at least one marker gene is situated within said transposon; b) integrating said transposon containing said chosen gene or said chosen DNA molecule into the chromosome or episome of said bacterium; c) activating said functional transposase gene by inactivating said temperature sensitive Mu repressor leading to expression of a functional transposase, whereby transposition of said transposon occurs: d) selecting said bacterial strain on a medium that is selectable for said marker gene in a manner such that a bacterial strain containing a specific number of copies of said chosen gene or chosen DNA molecule is obtained; and e) recovering the selected bacterial strain. 